Angmarean |
I like the idea and I'm planning on doing something a little similar in my campaign.
I'm toying with the idea that as part of the overall fungal growth the walls and floors themselves could be alive, at least in certain places. The lair could have originally been a lot more open plan than it appears with build up of fungus creating walls and floors that can be as fragile as I want them to be.
Fungus choked wellheads can give way as people cross them, alcoves containing traps/monsters/treasure/doors can be covered over with mold or vines. The denizens of the dungeon might use this to their advantage slipping in and out of areas that look too overgrown to allow passage.
It gives the impression that the dungeon itself is against the party, which kinda fits well with your idea of magic seeping out from the trapped mage and into his surroundings...